A New Cure for Screen Printing

New UV-curable screen printing inks provide comparable performance to conventional inks without the environmental issues and high energy requirements.

UV-curable screen printing inks give glass and ceramic decorators a new way to remain competitive in the container decorating market.

For many years, glass and ceramic decorators have wished they could enjoy the same advantages that the plastic decorators have come to take for granted—the ability to print multiple colors at high speed; in any color; free of heavy metals; and with simple, quick and low-cost curing. The latest regulations concerning the use of inorganic pigments, along with the recent energy concerns, have brought an even greater urgency to resolving these issues.

Recently, new ultraviolet (UV) screen printing inks* have been introduced that may hold the answer. These new UV systems allow users to print decorations in both glossy and satin finishes on a variety of products—from single use and returnable bottles, drinkware, mirrors and flat glass, to packaging for cosmetics, perfume and personal care products—with less environmental impact and considerably lower energy requirements than conventional screen printing inks.

Conventional Inks

Conventional systems often use a solvent to assist in the application of the product. The solvent controls the viscosity and thixotropic nature of the wet ink and keeps the ink in liquid form during processing. The curing process evaporates the solvent, leaving a stable ink film. Heat, an important part of the curing process, accelerates the reaction and increases the adhesion of the ink film to the glass substrate.

Figure 1. Drying behavior of a conventional screen printing ink.

Because a solvent-based ink contains about 30% solvent (a volatile organic compound [VOC]) by volume, the thickness of the ink film applied will be reduced about 30% after curing (see Figure 1). In many cases, solvent-based systems require two parts: the ink and a hardener. These parts combine and crosslink to become a strong, damage-resistant ink film.

With solvent-based systems, each successive color must be dried before another color is applied, and a final post-cure heat treatment is also required. Each of the drying and curing steps requires additional space and energy, making the process uneconomical for high-speed, multi-color operations. The maximum achievable speed using solvent-based systems is generally 40 prints per minute.

Thermoplastic screen printing ink is another conventional ink that has been used on glass and ceramic for decades. It is based on long-oil binder groups that are mixed with inorganic pigments, which usually contain heavy metals. During the printing process, the binder serves only as a pigment carrier. In the subsequent burn-in process, which generally takes place at over 600?C (1112?F) for more than 90 minutes, the binder is eliminated, and the pigment particles fuse with the glass or ceramic surface.

While curing between colors is not required with thermoplastic inks, the final curing or burn-in process can consume a great amount of space and energy due to the type of equipment and the duration required.

Figure 2. Curing behavior of a UV screen printing ink.

UV-Curable Systems

UV-curable systems contain no solvents or heavy metals. They are composed of monomers, oligimers, pre-polymers and photo-initiators that react when exposed to UV light. After UV curing—a process requiring no heat and therefore much less energy than conventional inks—a completely unique, hard and insoluble polymer is produced from the reactive raw materials. Because UV-curable ink contains no solvents, 100% of the ink applied remains after curing, resulting in the virtual elimination of VOCs (see Figure 2).

Despite their benefits, conventional UV-cure inks have not been durable enough to meet the high adhesion, scratch resistance and water resistance requirements of glass and ceramic decoration. Advances in the UV-cure system, however, have overcome this drawback. With the new UV-cure inks, a post-cure heat treatment (30 min/160?C [320?F] or 15 min/180?C [356?F]) is used to activate an ingredient in the ink mixture that results in a physical adhesion to the glass, rather than the purely mechanical adhesion that usually occurs with standard UV ink systems. Even with the post-cure process, however, the temperatures used are much lower than those required to cure conventional inks, and significant energy savings are still achieved.

The new UV screen printing inks exhibit the following characteristics:

Very good adhesion and scratch resistance on different glass and ceramic surfaces

High opacity (with the appropriate choice of screen mesh)

A high level of gloss or satin-finish effect

High dishwater resistance (300+ cycles in normal dishwashers)

Good alcohol resistance for use in the area of cosmetics

The results of specific tests in these areas can be seen in Table 1, and additional tests are ongoing. While each application has its particular requirements and requires specific testing, the performance of the new curable inks has so far been outstanding.

Components of a UV-Curable Ink System

UV screen printing inks consist of a mixture of monomers, oligomers and prepolymers. These liquid or semi-liquid plastics wet the pigments and are, among other things, responsible for the flow characteristics of the ink. The correct selection of binder components determines product characteristics such as adhesion to the substrate, degree of gloss and surface hardness, and chemical resistance.

The other main ingredients of UV ink are photo-initiators, pigments and additives.

Photo-initiators. The photo-initiators are responsible for the start of the radical polymerization. These decompose under UV light into their component parts and then into radicals, thus initiating polymerization. The concentration and type of photo-initiators used influences of the speed of hardening.

Pigments. Organic pigments are the only type of pigments used in the manufacture of UV screen printing inks. These pigments possess very high color intensity and brilliance. With their light-fastness and acid and alkaline resistance, the pigments determine the visual characteristics of the inks.

The finely milled pigments must be dispersed in the binder to maintain their full color strength. The incident light reflected by finely dispersed particles of optimum particle size allows the color intensity to be fully exploited. Light is not reflected in an ideal way on relatively large pigment particles.

Figure 3. The three roll mill pigment milling system.

To achieve the best possible results, the particle size of the pigments should normally not exceed 10 microns. This size reduction is normally accomplished through using a three-roll mill (see Figure 3).

Additives. The performance of the UV inks can be modified with the addition of additives, similar to conventional inks (see Table 2).

For the printer, these components, with the exception of additives, come pre-packaged and ready to use. This simplifies the use of the system since little if any ink formulation is required.

A Bright Future

UV-curable inks provide a number of advantages compared to conventional systems, including no heavy metals or solvents, lower energy requirements and faster curing speeds. Additionally, they are available in a wide range of colors and feature excellent chemical and abrasion resistance.

With the recent advances in UV-curable inks, today’s glass and ceramic decorators have a new option in screen printing—and a new way to remain competitive in the container decorating market.